You may expect velocity (and pressure) to increase with temperature. This is particularly true of old standby powders such as the IMRs, and there's a very nice correlation for them, actually (excluding extreme desert or arctic conditions).
HOWEVER, these days IMR 4198 is no longer produced in Canada at the old DuPont facility. It's now produced in Australia on the Thales line right alongside H4198. I don't say the burn rates are identical, but the IMR-labeled stuff is claimed to be 'extremely' temp-resistant now, just like the rest of the Thales-produced Hodgdon stick powders.
I don't know for sure why you saw what you saw, but I'll tell you this: Both ammunition _and_ rifle temperature drive pressure/velocity, particularly rifle temperature. Also, you can watch some of the 'extreme' powders actually slow down some as temps increase. I don't say that's what happened here, I just share it for information.